European Pat. No. 9239 describes the steam phase catalytic oxidation of compounds of the formula: ##STR2## wherein R is methyl, methoxy and t-butyl to produce the corresponding aldehydes using a complex molybedenum catalyst of the formula Mo.sub.12 Me.sub.a.sup.1 Me.sub.b.sup.2 Me.sub.c.sup.3 Me.sub.d.sup.4 Ox where Me.sup.1 is one or more of the elements Bi, Co, Ni, Fe, W, Nb, Ta; Me.sup.2 is one or more of the elements K, Rb, Cs, Tl, In; Me.sup.3 is one or more of the elements P, B, Sb, V, Sr, Cr; Me.sup.4 is one or more of the elements Mn, Re, Pd, Ir, Rh, Cu, Sn; Zn, Sm, Mg, Ce, Ag, Li, As, Ba, Ca; a=0-24 (if W.noteq.0.fwdarw.0,5-110); b=0.01-0.2 (if Bi, V or P&gt;0,5.fwdarw.0,1-6); c=0-6 (if a=o.fwdarw.0,5-5); d=0-10 and x is the number of oxygen atoms needed to satisfy the valencies of the other elements.